Preparation of organo metallic compounds



3,007,858 PREPARATION OF ORGANO METALLIC COMPOUNDS David G. Braithwaite, Chicago, 11]., assignor to Nalco Chemical Company, Chicago, 111., a corporation of Delaware No Drawing. Filed May 6, 1959, Ser. No. 811,262

. 8 Claims. (Cl. 204-59) This invention relates to the preparation of organo metallic compounds, and more particularly to a new and improved process for making tetraethyl lead.

The two principal methods which are employed to produce tetraethyl lead are described by the following equations which are designated, respectively, A and B:

ethyl lead. In actual practice, large quantities of lead must be recovered and reconverted to the lead-sodium alloy. 7

From a study of the literature it is apparent that electrolytic processes are not predictable. Krause and Von Grosse (Die Chemie der Metall-Organschen Verbindungen, Borntraeger, Berlin (1937), p. 23) reported that when a plate of metallic magnesium is used as the anode in the electroylsis of Grignard solutions magnesium metal is deposited on the cathode in the form of silvery plates which cover it in a loose layer. When a zinc anode is used zinc is deposited on the cathode as a thick layer in a secondary reaction. However, not all metals which were used as an anode dissolved upon electrolysis with formation of corresponding metallo organo compounds. Magnesium, zinc, cadmium and aluminum were dissolved. Platinum, copper, iron, lead, tin, cobalt, nickel and silver remain undissolved.

A process in which the anode is dissolved by electrolyzing is sometimes referred'to herein as a sacrificial anode process. 7

One of the objects of the present invention is to provide a new and improved sacrificial anode process for processing organo metallic compounds.

Another object of the invention is to provide a new and improved sacrificial anode process for producing organic lead compounds.

Still a further object of the invention is to provide a new and improved sacrificial anode process for producing tetraethyl lead. Other objects will appear hereinafter.

In accordance with the invention it has been found that organo metallic compounds can be produced by electrolyzing a substantially anhydrous solution of a Grignard 3,07,858 Patented Nov. 7, 1961 include organic chlorides, bromides and iodides. The halogen portion of the added organic halide does not have to be the same as the halogen portion of the Grignard reagent. The free hydrocarbon radicals derived from the Grignard reagent during electrolysis combine with the anode material to form the corresponding organo metallic compound which can be separated from the electrolyte in any suitable manner.

The cathode may be composed of a suitable conducting but non-reactive material such as platinum, stainless steel, graphite or other conducting material which does not dissolve in the electrolyte. In some cases the cathode may be composed of the same material as the anode. Thus, in producing tetraethyl lead both the cathode and the anode can betcomposed of lead. It is preferable, however, that the anode be composed of lead and the cathode of stainless steel.

The invention is particularly valuable in the preparation of tetraethyl leadand this preparation will be used to illustrate the practice of the invention. In carrying out this process a lead anode and preferably a stainless steel cathode are placed in a solution of ethyl magnesium chloride dissolved in a suitable organic solvent. A suitable organic solvent preferably employed for this purpose is the dibutylether of diethylene glycol. An electrolyzing current is passed into the ethyl magnesium chloride solution (Grignard solution) in sufiicient amount to cause the lead anode to be dissolved. Ethyl chloride is passed into the ethyl magnesium chloride solution either intermittently or continuously in sufiicient amount to react with the magnesium liberated at the cathode to reconvert it to ethyl magnesium chloride. The free ethyl radicals react at the anode with the lead to form tetraethyl lead. Magnesium chloride is a by-product of this process. The tetraethyl lead is removed in any suitable manner from the organic solvent solution. Where a high boiling solvent is used such as dibutyl ether of diethylene glycol, the tetraethyl lead is preferably removed by distillation. The residual solvent solution, before or after the removal of the tetraethyl lead, is treated to remove the magnesium chloride. ,This can be done by adding a substance which forms "an insoluble compound with the magnesium chloride, :for example, dioxane, and filtering the insoluble precipitate. The solvent solution from which the magnesium chloride has been removed is then recirculated to the cell in which the electrolyzing action is carried out or to a suitable container where it is used as a solvent for additional quantities of Grignard reagent. The removal of a partially electrolyzed solution from the electrolyzing cell can be carried out intermittently or con- 7 tinuously. The solution in the electrolyzing cell is prefreagent in an organic solvent for the Grignard reagent using a sacrificial anode and adding an organic halide to the electrolyte, the organic radical of which corresponds to the organic radical of the Grignard reagent being used. As the electrolyzing action proceeds, magnesium normally tends to deposit at the cathode and this normally causes serious problems, such as bridging between the anode and the cathode, but the added organic halide reacts with this magnesium and reconverts it to a Grignard reagent thereby avoiding the deposit of magnesium at the cathode. The term organic halide as used herein is intended to erably agitated with suitable mechanical stirring or other agitating means.

Theinvention will be illustrated but is not limited by the following examples.

Example I A solution of ethyl magnesium chloride in the dibutyl ether of diethylene glycol was charged into an electrolyzing solution in a stainless steel closed pressure cell having five stainless steel plate cathodes and six lead plate anodes spaced one-fourth inch apart, the anode and cathode areas each being .310 square cm. The normality ofthe ethyl magnesium chloride in the solution was 1.27 and the total number of moles of ethyl magnesium chloride charged were 2.15 so that the total amount of solution was approximately 1700 cc. A magnetic stirrer was provided beneath the electrodes to agitate the solution.

An electrolyzing current of 14 volts at an average amperage of 0.9 ampere was passed into the solution for thirty hours, then'24 volts starting at an amperage of 1.5

3 and dropping to 0.23 ampere in sixty hours. The temperature of the solution was 35-40 C. Ethyl chloride was added to the solution in a molar ratio of 0.9 mole of ethyl chloride per mole of ethyl magnesium chloride. At the end point when the conductance had dropped to about 0.3 ampere, analysis showed .69 mole of ethyl magnesium chloride remaining in the solution. Tetraethyl lead formed in the solution which separated into two layers. The percentage conversion to tetraethyl lead, based on consumption of ethyl magnesium chloride was 68%, and the percentage yield based on consumption of ethyl magnesium chloride was 100%. Substantially no gas was formed.

Example 11 The general procedure was the same as in Example I except that the normality of the ethyl magnesium chloride charged to the cell was 0.9. The total moles of ethyl magnesium chloride charged were 1.58. The molar ratio of ethyl chloride to ethyl magnesium chloride in solution was 2.5. The temperature of operation was 55-60 C. The electrolyzing action was carried out until approximately 63.5% by weight of the ethyl magnesium chloride had been converted. At this point .58 mole of ethyl magnesium chloride was recovered and .35 mole of ethane and ethylene had been evolved without taking into consideration the solubility of these gases in the solution. In this process the percentage yield of tetraethyl lead based on ethyl magnesium chloride was approximately 80.5 and the percentage by-product gas approximately 17.5. The current used was 12.5 volts, at an initial conductance of 0.47 ampere, rising in fifteen minutes to 0.6 ampere and continuing at 0.6 ampere for 76.5 hours.

Example Ill The electrolyzing process was carried out as in Example I except that the normality of the ethyl magnesium chloride charged was 1.0. The total moles of ethyl magnesium chloride charged were 1.75. The molar ratio of ethyl chloride to ethyl magnesium chloride in solution was 7.0. The temperature of operation was 65-85 C. The electrolyzing action was carried out until there was a 72.5% by weight conversion of the ethyl magnesium chloride. At this point 0.48 mole of ethyl magnesium chloride was recovered from the solution in the cell. The yield of tetraethyl lead based on ethyl magnesium chloride was about 73.5% by weight and the by-product gas amounted to 13.2% by weight. The voltage used in the foregoing process was 12 volts at a conductance of 0.15-0.18 ampere for 12 hours, rising to 2.3 amperes at the end of 14 hours, then dropping to 0.75 ampere after 24 hours, finally dropping to 0.36 ampere after 34 hours.

Example IV The procedure was the same as in Example I except that the normality of the ethyl magnesium chloride charged was 0.94. The total moles of ethyl magnesium chloride charged were 1.44. The molar ratio of ethyl chloride to ethyl magnesium chloride in solution was 7.4. The temperature of operation was 3338 C. The electrolyzing process was carried out until 85% of the ethyl magnesium chloride had been converted. The yield of tetraethyl lead based on ethyl magnesium chloride was 84% by weight and the yield of by-product gas was approximately 16% by weight. The voltage used was 14 volts at an amperage of .95 ampere for 45 hours, then 26 volts at an amperage decreasing from 0.9 to 0.13 ampere in 15 hours.

' Example V The process was carried out as described in Example I except that the dimethylether of ethylene glycol was used as a solvent, the normality of the ethyl magnesium chloride charged was 0.82, the total moles charged were 1.43, the molar ratio of ethyl chloride to ethyl magnesium chloride in solution was 1.0 and the temperature of operation was 50 C. to 65 C. The voltage of the electrolyzing current was 12 volts for one-half hour at 4.9 amperes dropping to 3.1, thereafter 26 volts with the amperage dropping from 3.1 to .05 amperes in 22 hours. A white solid separated from the clectrolyzing solution which appeared to be the etherate of magnesium chloride. The supernatant liquid was hydrolyzed in water to yield tetraethyl lead.

In this preparation the electrolyzing process was carried out until approximately 89.5% by weight of the ethyl magnesium chloride had been converted. Only about 1% by weight of gas was produced. The yield of tetraethyl lead was approximately 81% based on ethyl magnesium chloride. No extraneous lead metal was found suspended in the resultant mixture and in view of the small formation of by-product gas the lower than expected yield probably resulted from the fact that some of the ethyl magnesium chloride was carried out of solution with the magnesium chloride etherate that formed during electrolysis.

In carrying out the process as described in the examples it was noted that increasing the amount of ethyl chloride in the electrolyzing solution increased the conductance markedly and had the added advantage that no insolublelayer separated from electrolyzed solutions containing approximately 50% by weight ethyl chloride. Increasing the temperature also increased the conductance. However, as the ethyl chloride concentration was increased over the amount required to form ethyl magnesium chloride from magnesium metal in situ, the quantity of by-product gas also increased. In Example I the electrolyte separate into two layers. conductance had dropped to 0.3 ampere the cell was heated to C. which increased the conductance to 1.0 ampere. After a period of time, analysis indicated no increase in soluble lead compounds, but an increase in Grignard concentration. Upon examination of the electrodes there was a definite line indicating the position of the lower layer and on this portion of anodes was a definite deposit of PbCl This indicates that the complex lower layer conducts electricity at high temperatures, but the MgC1 rather than the EtMgCl, is decomposed which gives a slight increase in EtMgCl due to the magnesium formed at the cathode, but only gives PbCl at the anode.

While it is possible that many reactions may be reacting simultaneously, the general over-all reaction may be expressed by the following equations:

in which R represents the organic radical, X represents the halogen atom of the Grignard reagent, M represents the metal of the sacrificial anode which, in this case for the purpose of illustration, has a valence of 4, and Mg is the conventional symbol for magnesium.

Equation D shows why the optimum molar ration of organic halide to magnesium is approximately 1:1. From the foregoing examples it is apparent that as this ratio is increased by-product gases are formed, apparently due to the higher concentration of free radicals which can combine with each other and tend to do so rather than combine with the metal of the anode.

While'the invention has been illustrated with the preparation of tetraethyl lead it will be apparent that the metal M in equation C can be another metal which is capable of being electrolyzed in a Grignard reagent. Examples of such other metals are calcium, zinc, cadmium, manganese, mercury, lanthanum, thallium, arsenic, bismuth, tellurium and selenium. The radical R in equations C and D can be another organic radical which forms a Grignard reagent, for example, methyl, ethyl, propyl, isopropyl, butyl and higher homologues, phenyl, benzyl, and

After the the like. The radical X can be, for example, chlorine, bromine or iodine. Thus, other organic lead compounds or other organic metal compounds can be prepared by substituting other Grignard reagents for the ethyl magnesium chloride inthe foregoing examples and by using the corresponding organic halides in place of ethyl chloride. Specific examples of such other Grignard reagents are ethyl magnesium bromide, isopropyl magnesium chloride, isopropyl magnesium bromide, butyl magnesium bromide, amyl magnesium bromide, butyl magnesium chloride,.amyl magnesium chloride, and higher alkyl hom- :ologues. Similiarly, phenyl magnesium chloride, phenyl magnesium bromide, or mixtures of phenyl and ethyl :magnesium chloride or mixtures of phenyl and ethyl magnesium bromide can be electrolyzed to produce other organic lead compounds (or other sacrificial anode metals) containing the phenyl radical or both the phenyl and ethyl radicals or both phenyl and other alkyl radicals .in case a higher alkyl magnesium halide is substituted for the ethyl magnesium halide. In a similar manner benzyl magnesium chloride and benzyl chloride can be employed in electrolyzing a lead or other sacrificial anode. In carrying out the foregoing reactions it is preferable that the molar ratio of Mg(X) :RMgX does not exceed 2:1 becauseit has been noted that resin formation occurs if the Mg(X) content becomes too high. It is therefore desirable to control the Mg(X) content by removing Mg(X) in any suitable manner. This can be accomplished in some instances by using a specific solvent which will form an etherate with the Mg(X) as in Example V, or by adding dioxane which also forms a com- .plex insoluble substance, or by adding another material such as pyridine which forms a complex insoluble substance. The insoluble. complexes containing the Mg(X) canthen be removed from the electrolyzed solutioneither before or after separation of the organo metallic compound by filtration or other suitable means and the residual solution is returned for re-use in the electrolyzing cell.

The invention has'been operated over a wide range of current densities and at varying voltages. The spacing and size of the electrodes will determine the current density. Cells of the type used in the examples canbeoperated at a low voltage around 2.5 to '3 volts in 'which case the current density is around 10.01 ampere per square centimeter. Direct current voltages of I OO-and 300 volts "have also been used but, in general, it is desirable to use direct current voltages around 2 to 25 volts. 1

' The temperatures used can vary rather widely depending upon the type of cell employed, the solvent used, and the nature of the organo metallic compound. The process is normally carried out at temperatures above the freezing point of the solution and below the boiling points of the solvent and organo metallic compound. High current densities tend to heat the "solution and cooling may be applied, if necessary. In general, good results are obtained at temperatures Within the range of 20 C. to 85 C.

-In the exampleshigh boiling solvents for theGrignard reagent have been used. It is usually preferableto employ solvents which have a boiling point higher than the boiling point of the organo metallic compound which is being produced in the process. Thus, the dibutyl ether of diethylene glycol has a boiling point substantially higher than the boiling point of tetraethyl lead. This makes it possible to distil offthe tetraethyl lead and return the residual solvent to the electrolytic cell. In this way, the

, process can be carried out continuously by continuously removing a partially electrolyz'eds'olution from the cell, separating the tetraethyl lead by distillation and returning the residue. In order to prevent buildup of magnesium chloride in the cell, however, it is desirable, as previously indicated, to treat the electrolyzed solution before or after separation of the tetraethyl lead, with dioxane or another suitable compound capable of forming 6 an insoluble complex with the magnesium chloride so that the magnesium chloride can be separated from the solvent as a complex compound. An alternative procedure is to use a solvent which forms an insoluble precipitate with the magnesium chloride, thus making it possible to separate the magnesium chloride from the solvent which is recirculated to the cell.

The invention is not limited to any particular solvent except that the solvent must be relatively inert under the conditions of the process. For this purpose the solvent should not contain any labile hydrogen which is readily reactive. It is also desirable that the solvent have sufficient conductivity to permit passage of the current between the anode and the cathode. Solvents containing aliphatic hydrocarbon groups connected to oxygen atoms or nitrogen atoms are especially useful. Hydrocarbon solvents have poor conductivity and are less desirable. Low boiling solvents such as diethyl other can be employed but are difiicult to handle and require different methods for separating the organo metallic compounds therefrom. Solvents such as tetrahydrofurane can be employed. The process can be carried out with solvents for the Grignard reagent in which the'organo metallic compound is insoluble. Examples of suitable solvents are dimethyl ether, diethyl ether, diisopropyl ether, and higher molecular weight dialkyl ethers, including the others of polyoxyethylene glycols, polyoxypropylene glycols and polyoxyethylene-polyoxypropylene glycols which are liquid under the conditions of reaction. Special mention may be made of the dimethyl ether of diethylene glycol, the dipropyl ether of diethylene glycol, the dibutyl ether of diethylene glycol and the dimethyl ether of dipropylene glycol. Examples of solvents containing nitrogen are trihexylamine, triamylamine, pyridine and quinoline.

The pressure used in carrying out the process can also be varied and may be subatmospheric, atmospheric or superatmospheric. The pressures used in the electro lytic cell are normally suflicient to maintain the liquid phase with the particular solvent and temperature conditions employed. Where the organic halide is a relatively volatile halide, such as ethyl chloride, superatmospheric pressures in the cell normally prevail. For instance, in the examples pressures up to 50 pounds per square inch have been employed. The pressure used will also vary depending upon the quantity of the organic halide introduced into the solution to be electrolyzed. The type of solvent used is a major factor in determining the pressure.

In practicing the invention various procedures may be used. For example, in making tetraethyl lead using diethyl ether as a solvent, the operation is preferably conducted at elevated temperatures around C. and pressures sufiicient to minimize the formation of lead metal. The process can be operated until a considerable amount of tetraethyl lead is formed after which at least a part of the electrolyte is removed from the cell, some of the excess solvent is distilled, some of the desired product is separated and the residue comprising unreacted Grignard reagent and magnesium chloride etherate, together with some tetraethyl lead in the form of a slurry is recycled to the cell. An alternative is to electrolyze the cell to exhaustion before separating the end products.

' Using a solvent boiling above the boiling temperature of tetraethyl lead one can operate at elevated temperatures with ease either by employing pressure vessels to retain the ethyl chloride added or can add ethyl chloride continuously and collect the excess continuously at atmospheric pressure. As the reaction progresses at atmosphen'c pressure a point is reached where two layers form. Up to the point where the layer formation oc curs the formation of tetraethyl lead from the Grignard reagent is almost quantitative and the electrical efficiency is excellent. Electrolysis of the lower layer in ethyl chloride at low temperatures (25-30" C.) gives additional tetraethyl lead but at low electrical efliciency with the deposition of PbCl on the anode. As the electrolysis progresses magnesium chloride m-ay be precipitated from the reaction mixture by dioxane and removed by filtration. Thus, in the electrolysis of ethyl magnesium chloride in the dibutyl ether of diethylene glycol and ethyl chloride under pressure the electrolyzing can be carried out to near exhaustion of the Grignard, followed by separation of tetraethyl lead, solvent and MgC1 The invention makes it possible to prepare organo metallic compounds and especially substances such as tetraethyl lead by a very simple procedure. The process of the invention also makes it possible to produce such compounds in relatively high yields as compared to yields obtained by commercial processes.

By using more of the organic halide than is required to form the Grignard reagent the excess organic halide is available to react with magnesium that normally forms or deposits on the cathode. This not only makes it possible to remove the magnesium but at the same time re sults in the formation of additional quantities of Grignard reagent. Furthermore, it is no longer necessary to take precautions in order to prevent bridging of the magnesium between the cathode and the anode and it is possible to use cells in which the electrodes are relatively closely spaced. This in turn makes it possible to provide relatively large anode areas in a relatively small space.

The invention is hereby claimed as follows:

1. A process for preparing alkyl lead compounds which comprises electrolyzing, using a lead anode, a substantially anhydrous solution of a Grignard reagent in a substantially inert organic solvent for said Grignard reagent employing an electrolyzing current etfective to cause said lead anode to dissolve in said solution of said Grignard reagent in said organic solvent, adding an excess of an alkyl halide over that required for the formation of the Grignard reagent, and recovering from the resultant product an alkyl lead compound consisting of alkyl radicals linked directly to metallic lead.

2. A process for preparing tetraalkyl lead compounds which comprises electrolyzing, using a lead anode, a substantially anhydrous solution of a Grignard reagent in a substantially inert organic solvent for said Grignard reagent employing an electrolyzing current effective to cause said lead anode to dissolve in said solution of said Griguard reagent in said organic solvent, adding an excess of an alkyl halide to said solution over the amount required I to form said Grignard reagent, and recovering from the resultant product a tetraalkyl lead compound consisting of alkyl radicals linked directly to metallic lead.

3. A process for preparing tetraethyl lead which comprises electrolyzing, using a chemically inert cathode and a lead anode, a substantially anhydrous solution of ethyl magnesium chloride in a substantially inert organic solvent for said ethyl magnesium chloride employing an electrolyzing current effective to cause said lead anode to dissolve in said solution of said ethyl magnesium chloride in said organic solvent, adding about 0.9 mole of ethyl'chloride to said solution per mole of ethyl magnesium chloride, and recovering tetraethyl lead from the rmultant solution.

4. A process for preparing tetraethyl lead which comprises electrolyzing, using a chemically inert cathode and a lead anode, a substantially anhydrous solution of ethyl magnesium chloride in a substantially inert organic solvent for said ethyl magnesium chloride employing an electrolyzing current efiective to cause said lead anode to dissolve in said solution of said ethyl magnesium chloride in said organic solvent, said solvent having a boiling point higher than the boiling point of tetraethyl lead, adding extraneous ethyl chloride to said solution in an amount sufiicient to increase the conductance of the electrolyzing solution, separating tetraethyl lead from said solution and using the residual solution for further electrolyzing.

5. A process for preparing tetraethyl lead which comprises electrolyzing, using a chemically inert cathode and a lead anode, a substantially anhydrous solution of ethyl magnesium chloride in a substantially inert organic solvent for said ethyl magnesium chloride employing an electrolyzing current eifective to cause said lead anode to dissolve in said solution of said ethyl magnesium chloride in said organic solvent, said solvent having a boiling point higher than the boiling point of tetraethyl lead, adding extraneous ethyl chloride to said solution in an amount sufiicient to'increase the conductance of the electrolyzing solution, separating magnesium chloride from said solution, distilling tetraethyl lead from the residual solution and returning the residual solution after distillation of said tetraethyl lead for further use in the electrolyzing process. i

6. A process for preparingtetraethyl lead which comprises electrolyzing, using a chemically inert cathode and a lead anode, a substantially anhydrous solution of ethyl magnesium chloride in a substantially inert organic solvent for said ethyl magnesium chloride employing an electrolyzing current effective to cause said lead anode to dissolve in said solution of said ethyl magnesium chloride in said organic solvent, said solvent having a boiling point higher than the boiling point of tetraethyl lead, adding extraneous ethyl chloride to said solution in an amount sufiicient to increase the conductance of the electrolyzing solution, distilling tetraethyl lead from said solution, separating magnesium chloride from the residual solution and returning the residual solution from which the magnesium chloride has been separated for further use in the electrolyzing process.

7. A process for preparing tetraalkyl lead compounds which comprises electrolyzing, using a lead anode, a substantially anhydrous solution of a Grignard reagent in a substantially inert solvent for said Grignard reagent employing an electrolyzing current effective to cause said lead anode to dissolve in said solution of said Grignard reagent in said organic solvent, adding to said solution an alkyl halide in sufticient amount to increase the conductance of the clectrolyzing solution, and rextovering from the resultant product a tetraalkyl lead compound consisting of alkyl radicals linked directly to metallic lead.

8. A process for preparing tetraalkyl lead compounds which comprises electrolyzing, using a lead anode, a substantially anhydrous solution of a Grignard reagent in a substantially inert organic solvent for said Grignard reagent employing an electrolyzing current effective to cause said lead anode to dissolve in said solution of said Grignard reagent in said organic solvent, adding to said solution extraneous alkyl halide in a molar ratio of 0.9 to 7.4 moles of said alkyl halide per mole of said Grignard reagent, and recovering from the resultant product a tetraalkyl lead compound consisting of alkyl radicals linked directly to metallic lead.

References Cited in the file of this patent 

1. A PROCESS FOR PREPARING ALKYL LEAD COMPOUNDS WHICH COMPRISES ELECTROLYZING, USING A LEAD ANODE, A SUBSTANTIALLY ANHYDROUS SOLUTION OF A GRIGNARD REAGENT IN A SUBSTANTIALLY INERT ORGANIC SOLVENT FOR SAID GRIGNARD REAGENT EMPLOYING AN ELECTROLYZING CURRENT EFFECTIVE TO CAUSE SAID LEAD ANODE TO DISSOLVE IN SAID SOLUTION OF SAID GRIGNARD REAGENT IN SAID ORGANIC SOLVENT, ADDING AN EXCESS OF AN ALKYL HALIDE OVER THAT REQUIRED FOR THE FORMATION OF THE GRIGNARD REAGENT, AND RECOVERING FROM THE RESULTANT PRODUCT AN ALKYL LEAD COMPOUND CONSISTING OF ALKYL RADICALS LINKED DIRECTLY TO METALLIC LEAD. 